1. Field of the Invention
This invention relates to connecting devices for spinal fixation, in particular, for joining a bone anchor with spinal stabilizers such as a spinal rod or spinal plate in a polyaxial fashion.
2. Related Art
It is well know and practiced that when securing and maintaining bones in a preferred alignment, a plurality of bone anchors may be attached to one or more bones and subsequently secured together in a desired orientation with a bone stabilizer. Examples of bone stabilizers include “longitudinal members,” and in some cases, “connectors” as is known in the art. That is, the longitudinal member (e.g. a rod or a plate) connects the bone anchors together, and (if needed), the connectors are used to secure the bone anchor to the longitudinal member. Accordingly, there are at least two criteria that would be desirable to be satisfied when securing bones in a desired alignment using bone anchors and a bone stabilizer:    (i) it is desirable to attach each bone anchor to a bone in a manner that will inhibit movement of the anchor relative to the bone. Thus, the orientation of the bone anchor to its attached bone may be dependent on, e.g., the bone configuration, density, and/or fractures therein;    (ii) the bone stabilizer must be attached to each bone anchor, and in particular, to an included shaft, in a manner so that the longitudinal member (of the bone stabilizer) is oriented to effectively maintain the desired alignment of bones.
In order to satisfy the above criteria, it may be preferable that the shafts of the bone anchors are not parallel to each other and/or not perpendicular to the longitudinal member when they are attached to their respective bones. Moreover, for each bone alignment procedure performed using bone anchors and bone stabilizers, the orientations of the shafts relative to one another and the longitudinal member may be substantially unique.
Accordingly, a preferable position of the longitudinal member can be compromised by the various orientations of the bone anchoring shafts, these various orientations due to, e.g., the preferred positions of the anchors when they are secured to the bones. Thus, it is advantageous to have the ability to firmly secure such bone anchoring shafts to a bone position retainer, wherein the shafts may be at various orientations to the longitudinal member, i.e., in polyaxial or multi-angular orientations.
U.S. Pat. No. 5,984,924 discloses such a bone alignment system capable of multi-angular orientations with respect to the bone anchor and longitudinal member. Through use of a system of concave or convex arcuate washers, the multi-angular orientations are achieved.
U.S. Pat. No. 6,315,779 discloses a multi-axial bone fixation implant having an elongated member, one or more bone anchor assemblies, and stabilizer members which are fitted within the elongated member. The stabilizer members are rectilinear in shape and do not include arcuate surfaces. The only use of washers disclosed are ones having an undercut within its oblong aperture on the top of the implant. The implant is locked by a nut and the washer atop the elongated member, the enlarged portion of a bone anchor is forced against an inside wall of the stabilizer, which is in turn locks against the elongated member. Disadvantages of this system are the large number of separate parts and that the washers can be cumbersome to use in that they are not secured to the elongated member.
The present invention provides an advance in the art by minimizing a number of parts a surgeon has to handle during a surgical procedure while having the advantage of multiple orientations provided by use of the arcuate washers.